额外的传递是指什么，反正每回不管看英文还是中文myql文档都是晕乎乎的。

    *************************** 1. row ***************************

     Table: test_filesort

    Create Table: CREATE TABLE `test_filesort` (

     `a` int(11) DEFAULT NULL,

     `b` int(11) DEFAULT NULL,

     `c` int(11) DEFAULT NULL,

     KEY `a_2` (`a`)

    ) ENGINE=MyISAM DEFAULT CHARSET=latin1

    1 row in set (0.00 sec)

    No query specified

    +----+-------------+---------------+------+---------------+------+---------+-------+------+-----------------------------+

    | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

    +----+-------------+---------------+------+---------------+------+---------+-------+------+-----------------------------+

    | 1 | SIMPLE | test_filesort | ref | a_2 | a_2 | 5 | const | 1 | Using where; Using filesort |

    +----+-------------+---------------+------+---------------+------+---------+-------+------+-----------------------------+

    1 row in set (0.00 sec)

    Query OK, 6 rows affected (0.04 sec)

    Records: 6 Duplicates: 0 Warnings: 0

    +----+-------------+---------------+------+---------------+------+---------+-------+------+-------------+

    | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

    +----+-------------+---------------+------+---------------+------+---------+-------+------+-------------+

    | 1 | SIMPLE | test_filesort | ref | a_2,a | a | 5 | const | 1 | Using where |

    +----+-------------+---------------+------+---------------+------+---------+-------+------+-------------+

    1 row in set (0.00 sec)

    1.只有order by时可能会出现using filesort

    2.order by b，如果b列不在索引中，不管b值是否相同，总会出现using filesort

    possible_keys：指出MySQL可以使用哪个索引在该表中找到行

    key：显示MySQL实际决定使用的索引

    Using index:只从索引中检索数据，不回表

    key_len：显示MySQL决定使用的键长度

    rows：显示MySQL认为它执行查询时必须检查的行数

    ref：使用哪个列或常数与key一起从表中选择行。